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| Home > Publications database > Lidar observations of natural and volcanic-ash-induced cirrus clouds |
| Book/Dissertation / PhD Thesis | FZJ-2014-01691 |
2013
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-89336-847-1
Please use a persistent id in citations: http://hdl.handle.net/2128/5952
Abstract: Cirrus clouds - which consist solely of ice particles - influence the Earth’s radiation budget and thus the climate system since they scatter the incoming solar radiation (cooling effect) and absorb the outgoing radiation of the Earth (warming effect). However, this influence has not yet been quantified. The purpose of this thesis is to extend our knowledge about cirrus with the help of lidar measurements in combination with model simulations. In the first part, observations of cirrus clouds made with a backscatter lidar over Jülich are presented, focusing on obtaining a representative cirrus climatology. The cirrus clouds are evaluated with a temporal average extinction profile by considering a multiple scattering correction. By using additional meteorological data, the climatological observations are analyzed under macrophysical, radiative, and microphysical aspects and considered to be representative in comparison to other mid-latitude lidar climatologies. Most of the observed cirrus clouds were generated due to synoptic weather patterns (e.g. frontal systems), showing a large vertical thickness of about 2.2 km and occurring mostly directly below the tropopause. Mean and median values of optical depth are found to be around 0.28 and 0.12 (range: 0.002 - 3), respectively. The most frequently observed ice water content (IWC) was found to be 6 ppmv (range: 0.01 - 400 ppmv). The lowest detected IWC values from aircraft in situ measurements (∼ 0.01 ppmv) are confirmed by the lidar observations. This result is of importance, since up to now it has been unclear whether in situ aircraft instruments were able to detect the thinnest cirrus clouds. In the second part of the thesis, heterogeneous ice formation induced by volcanic ash from the Eyjafjallajökull volcano eruption in April 2010 is investigated based on a lidar observation of an ash-induced cirrus cloud in combination with model simulations along air mass trajectories. The microphysical properties of the cirrus cloud can only be represented by the microphysical model simulations on the assumption of an enhanced number of efficient ice nuclei (IN) originating from the volcanic eruption. The IN concentration determined by lidar measurements directly before and after cirrus cloud occurrence implies a value of around 0.1 cm$^{−3}$ (in comparison to normal IN conditions of 0.01 cm$^{−3}$). This leads to a cirrus cloud with rather small ice crystals having a mean radius of 12 μm and a modification of the ice particle number (0.08 cm$^{−3}$ instead of 3 · 10$^{−4}$ cm$^{−3}$ under normal IN conditions). The effectiveness of IN was estimated by the microphysical model and the backward trajectories based on ECMWF data, establishing a freezing threshold of around 105 % relative humidity with respect to ice in a temperature range from -45 to −55$^{\circ}$C. Only these highly efficient IN made the formation of the cirrus cloud in a slightly supersaturated environment possible. With the help of this case study, the possible influence of IN on the appearance and microphysical - and thus radiative - properties of cirrus clouds was demonstrated. From an additional set of idealized model simulations it can be concluded that the microphysical and thus optical properties of cirrus clouds are affected for the complete range of atmospheric conditions in the presence of a high number of IN.
Keyword(s): Dissertation
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